Non-aqueous electrolyte secondary batteries are a type of rechargeable battery in which ions move between the anode and cathode through a non-aqueous electrolyte. Non-aqueous electrolyte secondary batteries include lithium-ion, sodium-ion, and potassium-ion batters as well as other battery types.
Lithium-ion batteries are a popular type of non-aqueous electrolyte secondary battery in which lithium ions move between the cathode and the anode thought the electrolyte. The benefits and the challenges of lithium-ion batteries are exemplary of the benefits and challenges of other non-aqueous electrolyte secondary batteries and the lithium-ion example is not limiting. In lithium-ion batteries, the lithium ions move from the anode to the cathode during discharge and from the cathode to the anode when charging. Lithium-ion batteries are highly desirable energy sources due to their high energy density, high power, and long shelf life. Lithium-ion batteries are commonly used in consumer electronics and are currently one of the most popular types of battery for portable electronics because they have high energy-to-weight ratios, no memory effect, and a slow loss of charge when not in use. Lithium-ion batteries are growing in popularity for in a wide range of applications including automotive, military, and aerospace applications because of these advantages.
FIG. 1 is a cross section of a prior art lithium-ion battery. The battery 15 has a cathode current collector 10 on top of which a cathode 11 is assembled. The cathode current collector 10 is covered by a separator 12 over which an assembly of the anode current collector 13 and the anode 14 is placed. The separator 12 is filled with an electrolyte that can transport ions between the anode and the cathode. The current collectors 10, 13 are used to collect the electrical energy generated by the battery 15 and connect it to an outside device so that the outside device can be electrically powered and to carry electrical energy to the battery during recharging.
Anodes of non-aqueous electrolyte secondary batteries can be made from composite or monolithic anode materials. In composite anodes, particulate anode material is physically bound together with a binder forming a matrix of the particles and the binder. For example, anodes can be made from carbonaceous particles bound with a polymer binder. Monolithic anodes are anodes that are not made by the addition of a physical binder material. Any method of creating of a silicon anode where the silicon molecules are interconnected without the aid of an external binding agent is a monolithic film. Examples of monolithic anode materials include monocrystalline silicon, polycrystalline silicon and amorphous silicon. Monolithic anodes can also be formed by melting or sintering particles of anode material or by vacuum and chemical deposition.
During the charging process of the lithium-ion battery, the lithium leaves the cathode and travels through the separator as a lithium ion and into the anode. During the discharge process, the lithium ion leaves the anode material, travels through the separator and passes through to the cathode. Elements like aluminum, silicon, germanium and tin react with lithium ions and are used in high-capacity anodes. Anode materials that react with lithium have active areas in which lithium can react and inactive areas in which lithium cannot react. The ratio of the active to inactive area of the anode affects the efficiency of the battery.
In the reaction of lithium ions in a lithium-reactive material, there is a significant volume difference between the reacted and extracted states; the reacted state of lithium-reactive anode materials occupies significantly more volume than the extracted state. Therefore, the anode changes volume by a significant fraction during every charge-discharge cycle. In lithium-reactive anodes, cracks in the anode material are often formed during the cycling volume change. With repeated cycling, these cracks can propagate and cause parts of the anode material to separate from the matrix. The separation of portions of the anode from cycling is known as exfoliation. Exfoliation causes a decrease in the amount of active anode material that is electrically connected to the current collector of the battery, thereby causing capacity loss.
Exfoliation and degradation due to cycling are especially problematic for monolithic and particulate anodes comprising semiconductor anode materials on current collectors. Monolithic films and composite layers of semiconductor anode materials on current collectors degrade during cycling because of the significant expansion of the monolithic anode material at the interface of the anode material and current collector. A typical anode film is pinned to the current collector due to the deposition characteristics. Current flows through the collector and causes the layer of the anode material adjacent to the film to react first. After ions react with the anode, the conductivity of the reacted areas increases. This causes the anode material film to expand more rapidly in this region and causes stress to build up. The stress can result in rupture of the anode material film from the current collector. Further, monolithic films and composite films of semiconductor anode material on current collectors degrade during cycling because of stress caused by the mismatch between the current gradient and the ion concentration gradient in the system. For example, in a lithium-ion battery with a monolithic silicon anode, the lithium concentration on the silicon is the highest near the silicon-electrolyte interface. However, the silicon reactivity is the least at the silicon-electrolyte interface due to the inherent poor conductivity of the silicon material itself. This causes non-uniform lithiation of the silicon and results in film disintegration due to non-uniform mechanical stress distribution. The same problem occurs in composite anode films of semiconductor anode materials.
FIG. 1 shows the schematic of an anode 14 on a current collector 13. The anode 14 is contacted with an electrolyte containing lithium ions as part of a battery 15. If a semiconductor-containing anode is used in such a conventional electrode configuration, the anode atoms furthest away from the current collector 13 have the highest lithium concentration when the anode structure is charged with lithium. However, the anode atoms closest to the current collector 13 are the ones that are the most electrically reactive due to the anode's 14 high resistivity. This competition between the electrical activity (applied potential) and chemical activity (lithium concentration) forces uneven lithiation in the anode 14. The uneven lithiation causes stress, primarily at areas of defects, grain boundaries, and areas where the current collector 13, anode 14, and the electrolyte-containing separator 12 meet (at the ends of the film not shown in FIG. 1). This results in uneven expansion resulting in anode exfoliation and cracking from the current collector 13.